Newer Gen 4 carbon rails have a 1.8 lower, mine has the sameI agree that it doesn't sound right unless that's an aftermarket fork of some sort and then with some kind of an adapter headset for the bottom. All Rails have the 1.5 bottom and 1&1/8 top head tubes which is pretty much the industry standard for bikes at this level and category.
@bfletch2006 - if your existing fork is a zeb, you could leave the crown/steerer in place, and swap the internals and lowers of the new zeb ultimate onto it. And you can rebuild the leftovers into a 1.5 steerer fork which will be easier to sell on.